This invention relates to a precursor for cathode materials for rechargeable lithium batteries and a process to produce these precursors. The cathode materials are so-called NMC cathode materials, where NMC stands for nickel-manganese-cobalt. More particularly, this invention focuses on supplying precursors for NMC cathode materials with the aim that the final NMC cathode materials have a large surface area and therefore are particularly suitable for power demanding applications like batteries for hybrid electric vehicles.
NMC cathode materials are generally prepared by solid state reactions, wherein a source of lithium—for example Li2CO3—is blended with a Ni—Mn—Co containing precursor, and the mixture is fired in an oxygen containing atmosphere—for example air—to yield the final lithium transition metal oxide powder. Generally NMC has roughly the stoichiometry LiMO2, where M is a transition metal mostly consisting of Ni, Mn and Co. The crystal structure is an ordered rocksalt structure where the cations order into 2-dimensional Li and M layers. The spacegroup is R-3M. There are many different possible compositions, often categorized and named after their nickel, manganese and cobalt content. Typical NMC based materials are “111” where M=Ni1/3Mn1/3Co1/3, “552” with M=Ni0.423Mn0.423Co0.167, “532” with M=Ni0.5Mn0.3Co0.2, “622” with M=Ni0.6Mn0.2Co0.2, “261” with M=Ni0.222Mn0.667Co0.111, etc. In the current document, for simplicity, we will often refer to the metal composition by using the numbers, for example we will refer to M=Ni0.423Mn0.423Co0.167 as M=NMC 552.
Additional doping is possible, typical elements include Al, Mg etc. Generally, the Li to M stoichiometric ratio is near to—but often not exactly—unity. If Li:M increases Li replaces M on M-layer sites and the structure can—in a simplified manner—be written as Li1[M1−xLix]O2 or Li1+xM1−xO2, where Li:M=(1+x)/(1−x). Typical Li:M is about 1.10 for “111” and “442”, and 1.02 for “622”. One effect of increasing the Li:M stoichiometric ratio is that the cation mixing is changed. With cation mixing we mean that the real crystal structure is not exactly LiMO2 or Li1[M1−xLix]O2 but rather {Li1−xMx}[M1−yLiy]O2, where “x” refers to the M atoms on Li-layer sites, which thus undergo “cation mixing”.
NMC is a “mixed metal” cathode material, and it is known that NMC cannot be prepared from “non-mixed” precursors. The use of non-mixed precursors—for example NiO+Mn2CO3+Co3O4—generally results in a poor performance electrode material. In order that the cathode works well in the battery, within the Li-M-O2 crystal structure, the Ni, Mn, Co cations need to be well mixed at atomic scale. In the standard process, this is achieved by using mixed transition metal precursors, where the transition metal atoms are well mixed at nanometer scale. For NMC cathode preparation, usually a mixed metal hydroxide M(OH)2, or its oxidized form MOOH, is used as precursor. Mixed hydroxide precursors are usually prepared by a precipitation process. A process, which is widely used industrially, comprises a step where a flow of (a) a metal sulfate solution, (b) a NaOH solution and (c) a NH4OH solution are fed into a reactor. The resulting hydroxide contains sulfur, but is practically free of sodium. Most of the sulfur remains during the firing of the precursor and hence the final commercial NMC cathode contains sulfur. The standard precipitation process to prepare mixed hydroxide precursors involves the use of ammonia. The ammonia is a so-called chelating agent. The Ni-ammonia complexes increase the metal solubility and thus decrease the nucleation rate during precipitation. Without ammonia, for example, it would be difficult to prepare a sufficiently dense hydroxide, especially if large particles having sizes >10 μm are desired. Without ammonia, it is practically impossible to stabilize transition metal hydroxide precipitation conditions in a way, which yields large particles having the preferred spherical morphology. The ammonia that is present in a precipitation process always creates a certain safety risk. In the case of an accident, hazardous fumes evolve, so it would—from a safety point of view—be highly desirable to develop an ammonia free precipitation process. After precipitation, the ammonia remains in the filter solution. As the ammonia cannot be released to the environment, the waste water is treated to remove—preferably to recycle—the ammonia. These ammonia installations are expensive and increase the capital investment significantly, as well as the operating cost for the waste treatment, in particular by the higher need of energy. It would therefore be desired to develop an ammonia free precipitation method, which supplies mixed precursor having a sufficient density and spherical morphology, for reasons described below.
The use of a mixed metal carbonate as precursor for NMC has been reported also, but—to our knowledge—is not yet used industrially. The preparation of mixed metal carbonate precursors for lithium transition metal oxide cathode materials is known since a long time. For example, U.S. Pat. No. 7,879,266 discloses a mixed metal carbonate precursor having a particle size between 20 and 40 μm and a Brunauer-Emmett-Teller (BET) surface area between 50 and 130 m2/g. The tap density is above 1.7 g/cm3. The preparation is a co-precipitation of a dissolved transition metal salt with a carbonate or bicarbonate solution. The precipitation occurs at a CO3/M ratio of 2-10, preferably 3-8. U.S. Pat. No. 7,897,069 discloses a mixed metal carbonate precursor to prepare NMC. The particle size is 5-20 μm and the BET (Brunauer-Emmett-Teller) surface area is 40-80 m2/g. The tap density is above 1.7 g/cm3. The preparation is a co-precipitation of a dissolved transition metal salt with a carbonate or bicarbonate solution. The precipitation occurs at a CO3/M ratio of 2-7, preferably 3-6. The method of the patent uses an excess of carbonate (CO3) in the reaction solution and enables to achieve a composite carbonate with a high yield. However, if excess Na2CO3 is used the resulting carbonate has a high Na impurity and LiMO2 cathodes prepared from CO3 excess precursors show a poor performance. Other carbonate process patents are CN101229928 B, describing a carbonate precipitation process which includes ammonia, and U.S. Pat. No. 8,066,915 describes the corresponding process. U.S. Pat. No. 7,767,189 describes a process to prepare NMC which quite generally includes the precipitation of mixed metal carbonate. In the carbonate precipitation reaction, Na2CO3 is used, which is less corrosive than NaOH, and the pH during a carbonate precipitation is lower, that means less corrosive than that of a hydroxide precipitation. As a result, a carbonate process could more easily be implemented at mass production scale.
Other alternative precipitation methods include the bi-carbonate precipitation. It is relatively easy to achieve a mixed carbonate precursor with desired properties like spherical morphology, good density etc.
by the following bicarbonate process reaction:2NaHCO3+MSO4→Na2SO4+MCO3+H2CO3.  (1)
The disadvantage of this process is the low efficiency. To precipitate 1 kg of MCO3 typically about 1.5 kg of sodium bicarbonate is needed whereas a carbonate processNa2CO3+MSO4→Na2SO4+MCO3  (2)requires much less, only about 900 g of carbonate. Additionally, the solubility of bicarbonate is much lower (about 200 g/L at 90° C.) than the solubility of sodium carbonate (about 400 g/L). That means that compared with carbonate precipitation—the maximum yield of the bicarbonate process per liter of solution is 3 times lower, and this significantly increases the cost of filtering and wastewater treatment, rendering the bicarbonate process not fully competitive.
Compared to a carbonate precipitation a bi-carbonate precipitation happens at lower pH in the presence of a high concentration of carbonate. The lower pH tends to suppress Na insertion and the excess CO3 tends to suppress sulfur insertion into the mixed transition metal carbonate crystal structure. As a result, bicarbonate can allow to precipitate relatively poor transition metal carbonate.
It is generally desired to obtain pure MCO3 precursors for LiMO2 cathode preparation. A high impurity content tends to reduce the reversible capacity of the LiMO2 cathode due to the presence of the electrochemically “inert” second phase. So there tends to be consensus that sulfur is not desired, and especially a sodium impurity is harmful. The authors of the current patent application carefully investigated if impurities can be tolerated or even desired, and if yes, in what concentrations and mol ratios should impurities, especially sulfur and sodium, be present.
For automotive applications like Hybrid Electric Vehicles (HEV) high power batteries are needed. The cathode materials need to be able to support these high power rates. A major rate performance limitation is the solid state diffusion rate of lithium within a single particle. Generally the typical time for diffusion can be reduced—and thus a higher power can be achieved—if the solid state diffusion length decreases. The diffusion length can be decreased by reducing the particle size, but there are limitations since small particles have a low density. Such a low density is not desired because it causes problems during electrode coating, and the volumetric energy density of the final battery is low. Much more preferred are relatively large, spherical and relatively dense particles which have an open, interconnected porosity. In the present document we refer to a NMC cathode powder having large spherical, relative dense particles, but at the same time an interconnected meso or nano porosity as “NMC with preferred morphology”. The open, interconnected porosity contributes to the surface so “NMC with preferred morphology” has a much higher BET surface area than expected from dense particles having the same shape. So the BET surface area of commercial NMC consisting of dense particles is typically in the order of 0.2 to 0.4 m2/g. The NMC with preferred morphology typically may have a BET surface area in the range of 1 m2/g or higher. In the battery, the pores of NMC with preferred morphology will be filled with electrolyte, which acts as a diffusion highway into the particle because liquid diffusion is much faster than the diffusion in solid particles. Obtaining powders where the particles have this preferred morphology remains however a challenge. The present invention aims at providing NMC cathode materials and carbonate based precursors for those NMC cathode materials, the NMC cathode material being particularly suitable for use in automotive applications.